1. Field of the Invention
The present invention relates to a voltage regulator for generating a constant voltage based on an input voltage.
2. Description of the Related Art
In order to stabilize an output voltage, an output terminal of a voltage regulator generally includes an external capacitor. Before activation of the voltage regulator, the external capacitor does not store charges. During a time period from immediately after the activation to a moment at which the output voltage generates a constant voltage, a charging current flows into the external capacitor. The charging current generated at that time is in a state in which an equivalent impedance of the external capacitor is low, and hence an excessive rush current flows. In the worst case, the rush current may be a cause of breakage such as blowout of a wire bonding. Accordingly, the voltage regulator includes a circuit for limiting an output stage transistor with respect to the generation of the rush current.
Hereinafter, a conventional voltage regulator is described. FIG. 3 is a circuit diagram illustrating the conventional voltage regulator.
The voltage regulator includes an amplifier circuit 25, an output stage transistor T23 and a testing transistor T24, a switch circuit 30, a current limiting circuit 20, an on/off circuit 26, and a counter circuit 27. The amplifier circuit 25 compares a divided voltage obtained by dividing an output voltage of the voltage regulator by resistors R31 and R32 and fed back with a reference voltage generated by a reference voltage circuit to thereby control the output voltage of the voltage regulator. The output stage transistor T23 and the testing transistor T24 output drain currents corresponding to a voltage (gate voltage) output by the amplifier circuit 25. The switch circuit 30 selects an output destination of the drain current of the transistor T24. The current limiting circuit 20 controls gate voltages of the transistor T23 and the transistor T24 so that the drain currents have the detection current value or smaller in a case where the drain current of the transistor T24 has a predetermined detection current value or larger. The on/off circuit 26 controls on/off of the voltage regulator, and the counter circuit 27 counts an elapsed time from a moment at which the voltage regulator is turned on by the on/off circuit 26.
The current limiting circuit 20 includes an output current limiting circuit 21 for actually controlling an excessive drain current, and an output current limiting circuit 22 for controlling, by using a detection current value smaller than a detection current value of the output current limiting circuit 21, the excessive drain current. In the output current limiting circuit 21 and the output current limiting circuit 22, the counter circuit 27 controls the switch circuit 30 according to an obtained elapsed time. The switch circuit 30 connects the output current limiting circuit 22 to the transistor T24 until a predetermined elapsed time passes, and connects the output current limiting circuit 21 thereto after the predetermined elapsed time passes.
According to the above-mentioned voltage regulator, the on/off circuit 26 controls the voltage regulator to be turned on, the amplifier circuit 25 starts to operate, and the counter circuit 27 starts to count an elapsed time. Subsequently, the external capacitor connected to an output voltage terminal starts to be charged quickly, and hence the transistor T23 allows an excessive drain current (rush current) to flow. Based on the rush current, the transistor T24 allows a predetermined amount of the drain current to flow into the current limiting circuit 20. In this case, the switch circuit 30 is in a state of selecting the output current limiting circuit 22 which is likely to control the drain current. The output current limiting circuit 22 controls the gate voltages of the transistors T23 and T24 so that the drain current has the detection current value or smaller in a case where the drain current has a predetermined detection current value or larger, and controls an excessive drain current to be small. After the voltage regulator is tuned on and a predetermined elapsed time passes, the switch circuit 30 selects the output current limiting circuit 21 which is unlikely to control the drain current (for example, see 2003-271251 A).
However, in a voltage regulator including an overheat protection circuit, not only in a case where the voltage regulator is turned on and an excessive rush current is generated, but also in a case where the overheated state is detected and the output transistor is controlled to stop an output current and thereafter the temperature decreases and the output current starts to flow again, there is generated an excessive rush current that charges the external capacitor connected to the output terminal of the voltage regulator.